DEVICE AND METHOD FOR ELECTRIC DISCHARGE MACHINING

Abstract: An electric discharge machining device adapted to remove machined chips resulting from electric discharge machining by provision of an attraction device containing magnetic material in a work tank so as to eliminate a risk of machined chips penetrating into a gap formed between an electrode of a tool and a workpiece, a stable machining being thereby secured. In a case where a workpiece is machined by using an induction liquid containing powder material, a stirrer is provided for preventing settling of the powder material. This stirrer prevents machined chips from floating together with the powder material in the induction liquid without settling. The attraction device includes a plurality of long cylindrical magnetic materials and is preferably disposed so as to surround a workpiece from the side thereof.

Description: TECHNICAL FIELD The present invention relates to an electric discharge machine and method which uses a tool electrode to perform electric discharge machining on a workpiece. More specifically, the present invention relates to an electric discharge machine and method which use a machining fluid in which a powder is mixed into an oil-based dielectric fluid. BACKGROUND In general, electric discharge machines include a work tank which contains the workpiece, a fluid supply device which supplies a dielectric fluid such as kerosene to the work tank, and a power supply which supplies power to the space between the tool electrode and the workpiece known as the "gap." Electric discharge machining occurs at the gap, in work tank which is filled with the dielectric fluid. Technology is known whereby, for the purpose of improving surface roughness, a powder material composed of a semiconductor such as polycrystal silicon, or a good electric conductor such as aluminum or graphite, or an inorganic oxide or inorganic carbide, with a particle diameter of between 1µm and 50µm, is mixed into the dielectric fluid. On electric discharge machines which make use of such powder-bearing fluids, a fluid supply device is provided which can, in accordance with the intended purpose of machining, selectively supply either powder-bearing fluid or non-powder-bearing fluid. Such electric discharge machines generally also require two additional devices. First, a device is provided with separates machining waste produced by electric discharge machining from the powder-bearing fluid. For example, a large specific-gravity centrifugal separator may be provided for this purpose. Also, a device containing a magnetic material is provided in an appropriate part of the fluid supply device, such as the fluid reservoir tank or fluid circulation path. Second, because it is important from the standpoint of machined surface quality and machining performance to maintain the concentration of powdered material in the dielectric fluid in the work tank at a specified concentration, a device is provided which prevents settling of the powdered material. Typically an agitator is provided in the work tank to agitate the fluid. The agitator prevents the settling of powdered material by agitating the fluid in the work tank, but at the same time it inhibits the settling of machining chips produced by the machining process. The agitator can also cause machining chips which had settled to become resuspended in the fluid. As a result, the machining chips suspended in the fluid can enter the machining area, where it will have undesirable effects on the electric discharge machining process. EP-A-0 098 711 discloses an apparatus in which agitators are provided for blending liquid components, for separating surface active agents and for separating and sedimenting solid components. DE-A-3 530 536 discloses a magnetic device for attracting metal chips which are generated during electric discharge machining. The purpose of the present invention is to provide an electric discharge machine and method which reliably removes machining chips suspended in the fluid by the agitator. Other purposes, advantages, and new characteristics of the present invention are partially discussed in the explanation which follows below, and will be partially apparent to practitioners of the art upon consideration of the explanation below, or practitioners of the art will be able to learn of these through application of the invention. The purposes and advantages of the present invention will be realized and achieved by the means and instrumentalities set forth in the appended claims. DISCLOSURE OF THE INVENTION According to the present invention there is provided an electric discharge machine for machining a metal workpiece by electric discharges in a machining gap formed between the workpiece and a machining electrode comprising: a work tank for containing a machining fluid in which said machining gap is submerged; a machining fluid supply for supplying machining fluid containing a powdered material into the work tank; and an agitator positioned in the work tank for agitating the machining fluid throughout the work tank to prevent settling of the powdered material, said machine being characterized by; a magnetic adsorption filter positioned in the work tank to adsorb metal chips produced by the electric discharges and suspended in the machining fluid. The magnetic adsorption device will preferably be positioned so that it faces the flow of dielectric fluid produced by the agitator in the work tank. The magnetic adsorption device may also be positioned so that it surrounds the workpiece about its sides. It is preferable that the magnetic adsorption device include multiple magnetic pieces and retainers which maintain a parallel interval between those magnetic pieces. It is also preferable that the multiple magnetic pieces be formed in a cylindrical or flat shape. According to the present invention in a second aspect there is provided a method of machining a metal workpiece by means of electric discharges using a tool electrode, the workpiece being positioned in a work tank containing a machining fluid, said method comprising the steps of: (a) generating electric discharges at a machining gap formed between the workpiece being shaped and the tool electrode thereby producing machined chips; (b) supplying machining fluid containing a powdered material into the work tank; and (c) agitating the machining fluid throughout the work tank to prevent settling of the powdered material, said method being characterized by the step of: (d) magnetically adsorbing the metal chips produced by the electric discharges and suspended in the machining fluid. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overview diagram illustrating an exemplary embodiment of an electric discharge machine according to the present invention. FIG. 2 is an external view illustrating another example of a machining chip adsorption device. FIG. 3A is a section depicting an example of the magnetic pieces in a machining chip adsorption device. FIGS. 3B and 3C are sections depicting another example of the magnetic pieces in a machining chip adsorption device. FIG. 3D is an external view depicting still another example of the magnetic pieces in a machining chip adsorption device. OPTIMUM CONFIGURATION FOR IMPLEMENTING THE INVENTION The construction of the present invention will now be described with reference to the drawings. FIG. 1 is an overview drawing illustrating an embodiment of the electric discharge machine according to the present invention. The electric discharge machine includes a bed 7 and an XY cross-table 4 attached to the bed. Work tank 3 is fastened to the XY cross-table 4 in such a way that it can be filled with fluid. A column 6 stands upright on the bed 7, and is positioned behind the work tank 3. A servo-head 5 is attached to the column 6. A tool electrode 1 is fastened to the servo-head 5 by an appropriate electrode holder and faces the workpiece 2, which is affixed to the XY cross-table 4. The servo-head 5, includes a Z-axis motor, moves the tool electrode 1 in a direction perpendicular to the XY cross table. The electric discharge machine further includes a combination power supply/control unit 10 which supplies power to the gap formed between the tool electrode 1 and the workpiece 2. A control unit 10 controls the X-axis motor and Y-axis motor (not shown) which drive the XY cross-table 4 and cause the workpiece 2 to move relative to the tool electrode 1 in the horizontal plane. The control unit 10 also controls the Z-axis motor. The electric discharge machine further includes a fluid supply unit 20, which supplies a mineral oil-based machining fluid to the work tank 3 through a flexible tube 8. A flexible tube 9 is provided which discharges the fluid from the work tank 3 to the fluid supply unit 20. The fluid supply unit 20 includes a fluid supply section 21, which stores non-powder-bearing dielectric fluid F1 and supplies the fluid F1 to the work tank 3, and a supply section 22, which stores powder-bearing dielectric fluid F2 and supplies the fluid F2 to the work tank 3. The fluid supply unit 20 also includes a switch 23 which selectively supplies the fluids F1 or F2 to the work tank 3 and selectively returns fluid discharged from the work tank 3 to the fluid supply units 21 or 22. The switch 23 is controlled by a signal from the control unit 10. An agitator 30 is provided in order to prevent the settling of the powdered material. The agitator 30 includes a drive motor and an agitation blade attached to the end of the driveshaft thereof. The agitator 30 may, for example, be attached to the side wall of the work tank 3 using an appropriate fastener. In the illustrated embodiment, the agitation blade is formed in a propeller shape, and is positioned within the fluid-filled tank. A drive source 31 is connected to the drive motor and is activated in response to a signal from the control unit 10. An appropriate number of additional agitators may also be erected in order to reliably prevent the powdered material from settling. A machining chip adsorption device 40 which contains, for example, multiple magnetic pieces such as permanent magnets or electromagnets, is also provided in order to separate the machining chips produced by electric discharge machining. These magnetic pieces may be formed as cylinders having a diameter of approximately 1∼2 cm, and there are multiple magnetic poles on the perimeter surface thereof. The adsorption device 40 also has retainers which suspend the magnetic pieces from their top ends in such a way that the multiple magnetic pieces are arrayed in a line perpendicular to and at an interval from the XY cross table 4 and parallel to one another. The magnetic pieces in a row are suspended in the work tank 3 in such a way as to cross the fluid flow stream produced by the agitator 30 and in a such a way that the bottom ends of the magnetic pieces virtually reach the bottom of work tank 3. When the adsorption device 40 includes electromagnets, a controller 44 is provided to energize and denergize the electromagnets. FIG. 2 is an top view of work tank 3. It illustrates an embodiment of an electric discharge machine provided with multiple adsorption devices. In the illustrated embodiment, plural machining chip adsorption devices 401, 402, 403, and 404 are positioned on the XY cross table 4 in the work tank 3 in such a way as to surround the workpiece 2 about its sides. Parts identical to those shown in FIG. 1 are assigned the same reference numerals, and an explanation thereof is omitted. The adsorption device 401 includes multiple cylindrical electromagnets 401a, which are of sufficient length that they virtually reach from the bottom of work tank 3 to the fluid surface, and retainers 401b, which hold the lower ends of the electromagnets 401a and are arrayed on the XY cross table 4. The magnetic pieces 401a are, for example, electromagnets with cores formed of laminated silicone steel plate. The electromagnets 401a are held parallel to one another and perpendicular to and at an interval with respect to the XY cross table 4. Electromagnets 401a may, as with the adsorption device 40 shown in FIG 1, be suspended using appropriate retainers to hold the top ends thereof. In order to surround workpiece 2 about its sides, the electromagnets 401a may also be arrayed parallel to one another and parallel to and at an interval with respect to the XY cross table 4 using cable or wire so as to form a folding curtain. Preferably, multiple magnetic pieces 401a will be arrayed so that a magnetic field is generated parallel to the axes of the magnetic pieces and, further, so as to form some degree of magnetic field between the magnetic pieces themselves. The other adsorption devices 402, 403, and 404 are typically of the same construction as the adsorption device 401. FIGS. 3A, 3B, 3C, and 3D are section drawings depicting examples of the magnetic pieces of a machining chip adsorption device. The magnetic piece 410 illustrated in FIG. 3A has multiple electromagnets 411, 412, 413, and 414 connected by a non-magnetic body. The electromagnet 411 includes a rotating I-shaped core 411a, a coil 411b wound around the trunk of the core 411a, and a non-magnetic body 411c. A magnetic field is formed at both ends of rotating I-shaped core 411a, and also between the opposite poles of an electromagnet 412 which, interposed by a non-magnetic body, adjoins it. The core 411a may also be formed in a hollow cylindrical tube shape, with rod-shaped permanent magnets inserted into the hollow portion thereof. In this case, the hollow gap around the core 411a will be filled with a non-magnetic body such as plastic rather than with the coil 411b. A cylindrical part formed of thin stainless steel or plastic is fashioned, into which magnetic piece 410 is inserted. Machining chips adsorbed on the outer surface of the cylindrical part can be easily peeled off when magnetic piece 410 is extracted from this cylindrical part. The elongated cylindrical magnetic piece 420 illustrated in FIGS. 3B and 3C includes a cylindrical piece 420a comprising a non-magnetic body. The magnetic piece 420 further includes plural cores 421a, each comprising a squared-off "C"-shaped silicon steel plate laminate -- in the example illustrated, 6 such cores 421a are radially arrayed in 60 degree divisions around the axis of the cylindrical piece 420a as depicted in FIG 3C. A coil 421b is wound around each of the plurality of 6 cores 421a to form an approximately disk-shaped electromagnet in which the lines of magnetic force extend radially. Several of these electromagnets may be stacked up along the aforementioned axis to constitute an elongated rod shape, and an appropriate non-magnetic body may be used to fill the spaces between the cores 421a and the electromagnets and inserted into the cylindrical piece 420a. Alternatively, a permanent magnet constitution may be adopted in which permanent cylindrical magnets are inserted coaxially into the bottom of the 6 cores in place of the coil 421b. In the elongated square pillar-shaped magnetic piece 430 illustrated in FIG. 3D, multiple permanent magnets 431, 432, 433, etc. are positioned within a casing piece 430a in such a way that magnetic poles are generated on the side walls of the elongated rod-shaped body. This assembly is inserted into a thin-walled container, and removal of adsorbed machining chips is accomplished by extracting the magnet 430 from the aforementioned container. A columnar shape using electromagnets was adopted in the embodiment of FIG. 2, but as shown in FIG 3D, the device may also be formed as an elongated square pillar, plate, or columnar shape, using multiple small permanent magnets internally and covering them with resin, etc. Next the operation of the electric discharge machine depicted in FIG. 1 will be described. First, after affixing the workpiece 2 to the table 4, fluid F1 is supplied to the work tank 3 in accordance with a command signal from the control unit 10, and the workpiece 2 is submerged in the fluid F1. A course removal operation is then performed on the workpiece 2 under the requisite machining conditions. Multiple machining conditions may be sequentially changed as required to gradually finish the workpiece to the requisite machining dimensions and machining surface roughness. In this rough removal process, the fluid F1 is supplied constantly from the fluid supply section 21 to the work tank 3; the fluid F1 in the work tank 3 overflows, forming a relatively slow machining fluid flow which returns to the fluid supply section 21. By jetting the fluid F1 into or suctioning it from the gap through the path formed in the tool electrode 1 or the workpiece 2, machining chips produced at the gap formed between the tool electrode 1 and the workpiece 2 as a result of electric discharge machining are removed. The fluid F1 can also be jetted into the gap using a flexible hose equipped with a nozzle. Generally, in order to remove machining chips from the gap, a so-called "pumping" process is effected in which electrode 1 is cyclically moved a specified distance in a direction away from the workpiece 2 and then immediately moved back into proximity with the workpiece 2. Upon completion of the rough machining process, the work piece 2 is finished using the fluid F2. First, the fluid F1 is discharged from the work tank 3 to the fluid supply section 21 through the tube 9. In accordance with a command signal from the control unit 10, the fluid F2 is supplied from the fluid supply section 22 to the work tank 3 through the tube 8, and the workpiece 2 is submerged in the fluid F2. When the fluid F2 is supplied to the work tank 3 and the fluid surface reaches a specified level, a signal is sent from a fluid surface detector (not shown) to the control unit 10. The control unit 10 drives the agitator 30 while at the same time turning on the power supply in controller 44, so that a magnetic field is generated on the surface magnetic pole portion of adsorption device 40 and an adsorbing magnetic field is formed. When the agitator 30 agitates the fluid F2 in the work tank 3, the machining chips which had settled as residue in the work tank 3 are churned up by the agitation-induced fluid flow, and become suspended in the fluid in the work tank 3 along with the powdered material. When the suspended machining chips start to pass between the magnets of the adsorption device 40, they are attracted by the magnetic field present in those spaces and are adsorbed on the surface magnetic polar portions of the magnetic columns. Therefore the machining chips which create the difficulties described above will not become interposed in the machining area. The workpiece 2 can be machined under requisite machining conditions in such a way that required dimensional accuracies and surface roughness are achieved. The adsorption device 40 should preferably be activated during the finishing processing so as to remove machining chips suspended in the fluid F2. The purpose of the present invention is not limited to the precise disclosed forms. Clearly there are many improvements and variations possible in light of the above teachings. For example, a die cutting electric discharge machine was depicted in the embodiment of the present invention, but the present invention can also be used in a wire-cut electric discharge machine. The above embodiment was selected for the purpose of explaining the essence and practical application of the invention. The scope of the invention is defined in the appended claims.

Claims: An electric discharge machine for machining a metal workpiece (2) by electric discharges in a machining gap formed between the workpiece and a machining electrode (1) comprising: a work tank (3) for containing a machining fluid in which said machining gap is submerged; a machining fluid supply (20) for supplying machining fluid containing a powdered material into the work tank; and an agitator (30) positioned in the work tank for agitating the machining fluid throughout the work tank to prevent settling of the powdered material, said machine being characterized by; a magnetic adsorption filter (40) positioned in the work tank to adsorb metal chips produced by the electric discharges and suspended in the machining fluid. The electric discharge machine of Claim 1 wherein said agitator creates a flow of machining fluid in the work tank and said magnetic adsorption filter is positioned facing the flow of the machining fluid. The electric discharge machine of Claim 1 wherein said magnetic adsorption filter surrounds the workpiece being machined about its sides. The electric discharge machine of Claim 1, 2 or 3 wherein said magnetic adsorption filter includes a plurality of magnetic pieces (401a) and retainers (401b) for maintaining the magnetic pieces spaced apart and parallel to one another. The electric discharge machine of Claim 4 wherein each of said plurality of magnetic pieces comprises a generally columnar shape. The electric discharge machine of Claim 4 wherein each of said plurality of magnetic pieces comprises a generally flat shape. A method of machining a metal workpiece (2) by means of electric discharges using a tool electrode (1), the workpiece being positioned in a work tank (3) containing a machining fluid, said method comprising the steps of: (a) generating electric discharges at a machining gap formed between the workpiece being shaped and the tool electrode thereby producing machined chips; (b) supplying machining fluid containing a powdered material into the work tank; and (c) agitating the machining fluid throughout the work tank to prevent settling of the powdered material, said method being characterized by the step of: (d) magnetically adsorbing the metal chips produced by the electric discharges and suspended in the machining fluid.

Inventors: FUJIWARA KATSUHIDE, FUJIWARA, KATSUHIDE
Applicants: SODICK CO LTD, SODICK CO., LTD.
Application year: 1994
Main classification: B23H1
IPC codes: B23H1, B23H7